The present invention relates to image pickup apparatus, and more particularly relates to an image pickup apparatus having a function for correcting false signals.
In conventional solid-state image pickup apparatus using CCD image pickup device, there is a problem of image quality deterioration due to smears which occur as a phenomenon peculiar to CCD image pickup device when an intense light has been incident thereupon. In particular, such phenomenon occurs as charge not captured at its true charge accumulating region is caused to leak into the vertical transfer line across the potential barrier. If caused by an ordinary and steady light, such phenomenon occurs over a vertical transfer period. In the case of a spot light, it results in a vertical stripe extended up and down therefrom. Also, if the image object includes a horizontally extended bright portion such as a scenic view containing the sky, a flare-like whitish image over the frame as a whole is the outcome.
A method for example as disclosed in Japanese patent application laid open Hei-7-67038 has been known as the method for correcting such phenomenon. Noticing the fact that a smear occurs substantially at the same level along the vertical direction due to the principle of its occurrence, the output level of vertical 0B (optical black) pixels, i.e., an optically concealed accumulating pixel region outside the effective pixel region, is determined as a smear eliminating reference signal which is subtracted from the pixel output signals of an effective frame.
Specifically, the smear eliminating reference signal as disclosed in the same laid-open application is defined as line information (mean value data corresponding to pixels of one line) obtained by means of addition as mean values (the respective mean of every 12 vertical pixels) of signals of vertical 0B pixels consisting of a plurality of lines (12 lines in an embodiment thereof), so as to reduce the effect of randomized noise contained in such reference signal. Hence a consideration is made in order not to cause an additional fixed pattern noise to the contrary due to the smear correction.
Further, special drive methods are sometimes used in image pickup apparatus using CCD image pickup device. As a typical example of special drive, a description is given below with respect to a known “n-multiple rate vertically added drive (n-addition drive)” which is a high-speed, high-sensitivity readout drive. In this drive method, the number of pixels (transfer clock number) to be transferred to H (horizontal) transfer line from V (vertical) transfer line during each one H (horizontal) blanking period is set to a natural number n of two or more instead of the usual “1”. The number of vertical lines corresponding to one frame is thereby set to 1/n. As a result, the readout time of one frame becomes 1/n, and the sensitivity is increased to be n times greater by charge addition at the time of transfer.
It should be noted that, as a further developed form of this drive method, “m/n-addition drive” for selectively transferring only specified m (≦n) lines out of n lines to be added at the time of transfer from the vertical transfer line to the horizontal transfer line (including “n-addition drive” as a special case, i.e., “n/n-addition drive” where m=n) is also used in charge transfer to the V transfer line from the signal accumulating region of the pixels before V transfer, for the purpose of considering color coding pattern in a color image pickup device or of suitably adjusting the sensitivity. However, since these are essentially identical as far as the explanation of the present invention is concerned, this specification will explain the case of m=n, i.e., the above described n-addition drive as the representative of these.
Since the smear correction method described above as a prior-art technique premises a normal drive, it cannot be applied to an image pickup apparatus for performing special drive such as the above described n-addition drive. Specifically, suppose that subtraction and correction are made by obtaining a smear eliminating reference signal as “mean value data corresponding to pixels of one line” as disclosed in the above laid-open application. Since the smear component contained in the output signals of n-addition drive corresponds to “n” lines due to addition at the time of transfer, the smear component corresponding to “n-1” lines remains as a result without being eliminated. Further, if pixel defect occurs in the vertical 0B pixels, such information is contained also in the smear eliminating reference signal. Hence there is another problem that additional fixed-pattern noise is newly caused to the contrary due to the correction.